High speed, small signal sensing amplifiers are commonly needed in memory such as Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM). Small signal sensing circuits are used because the current trend is toward small signal transmission over wide data buses either external or internal to a semiconductor chip. Small signal transmission is being more widely used because switching power is proportional to CV2ƒ , where C is the capacitance, V is the voltage, and ƒ is the frequency of switching. The frequency used for switching on buses has constantly increased, as the frequency is related to how much data per unit time can be shuttled through the buses. To limit power, the voltage has been decreasing too. Thus, small signals are being transmitted through buses, and these small signals require small signal detection and amplification at the receiving end. For example, voltages under one volt are being used as the high logic level for small signals. The small signal sensing circuits can operate in single ended mode, namely using only one signal relative to ground (GND) or supply voltage (VDD). The small signal sensing circuits can also operate in differential signal mode, using a pair of differential signals.
In conventional single ended, small signal sense circuits such as class A amplifiers, there are a number of items that are very difficult to control: biasing of the operating point; stability of the reference voltage; biasing current; sensitivity to threshold voltage; and process and temperature variations. This is especially true for future technology, due to increasing high leakage current and low supply voltage as transistors are scaled smaller, making such circuits very sensitive to voltage, temperature and process variations. For conventional differential sense circuits, due to the increasing statistical variation between adjacent transistors in future technology, the advantage of differential mode small signal sensing is diminishing.
Thus, there is a need to provide improved amplifiers for uses such as signal sensing.